demux: mp4: fix reading stsd v2 extensions
demux: mp4: fix reading stsd v2 extensions

miscomputed offset breaking audio extensions
like channels mapping.

regression refs #12002 sample

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc